The update, which is the first in 27 years, stems from mounting evidence that the degeneration of nerves deep within the brain starts years or even decades before memory loss and other cognitive changes are noticeable.

"It is our hope that incorporating scientific knowledge gained and technological advances made over the past quarter century will improve current diagnosis, bring the field closer to earlier detection and treatment, and ultimately lead to effective disease-modifying therapies," Alzheimer's Association chief medical and scientific officer William Thies said in a statement.

The new criteria, published today in Alzheimer's & Dementia: The Journal of the Alzheimer's Association, open the door for research into the earliest stages of the disease and the development of drugs that may slow or stop the degenerative process before the damage is done. Their release follows a report that brain areas affected by Alzheimer's disease start shrinking up to a decade before symptoms appear.

"We at least have the potential to detect changes a number of years in advance, and hopefully we could do something about it," said lead author of the April 13 study Dr. Bradford Dickerson, associate professor of neurology at Harvard Medical School and director of the Frontotemporal Dementia Unit at Massachusetts General Hospital in Boston.

The new criteria detail three stages of Alzheimer's disease: preclinical (before outward symptoms are visible); mild cognitive impairment (mild memory and thinking changes enough to be noticed but not debilitating); and dementia, or full-on Alzheimer's disease.

Some researchers call the diagnostic criteria revamp a necessary step -- one they hope will breathe new life into a field deflated by a string of negative drug trials.

"Most of current therapies are aimed at amyloid or tau [proteins], and the thinking is that they may work best at very early stages," said Dr. Murali Doraiswamy, chief of biological psychiatry at Duke University Medical Center in Durham, N.C.

Doraiswamy said variability among patients enrolled in clinical trials makes it difficult to detect small drug effects.

"At least 20 percent of patients in trials are misdiagnosed, plus there is great variability among patients in rates of decline, which affects the power of a trial to test drug effects," Doraiswamy said. "The new criteria will hopefully help increase diagnostic accuracy and homogeneity in clinical trials."

New Alzheimer's Disease Test Criteria Not Ready for Prime Time

But the tests used to spot features of preclinical Alzheimer's disease, many of which are still under development, are not yet approved by the Food and Drug Administration as diagnostic or prognostic tools, which means the new criteria will currently only be used for research purposes. And in the future, the cost of such tests and the meaning they carry will have to be carefully considered.

"We don't have any problem understanding that atherosclerosis may or may not lead to a heart attack, for example, but the idea of Alzheimer pathology is much scarier for now, in part because we don't know what it means for prognosis," said Dr. Norman Foster, director of the University of Utah Center for Alzheimer's Care, Imaging and Research in Salt Lake City.